A brief history of atomic time

Our concept of time is based on what we see around us - the planets and stars - but, says Fiona Auty, to be really useful, time needs to come from a somewhat smaller source

The NPL Strontium optical clock, 2005.

ON 3 June 1955 scientist Louis Essen, working at the National Physical Laboratory (NPL) in Teddington, Middlesex, pencilled columns of numbers and hand-drawn graphs into his laboratory book, to be written up at a later date. Using an idea first proposed by Lord Kelvin in 1879, this was the first confirmation that a clock based on atomic structure could keep time more accurately than any method based on the movement of celestial bodies.

Since Essen's first demonstration the atomic clock has been developed and refined almost beyond recognition and its diverse applications permeate our daily lives. In 1967 atomic clocks allowed the second to be redefined from solar time to atomic time. The best clocks today are over 100,000 times more accurate than Essen’s clock, keeping time to within a tenth of a billionth of a second a day. The second is the most accurately realised of the SI units, and the first to be defined using a quantum standard (the only SI unit still defined by a physical artefact is the kilogram). There are now thousands of atomic clocks across the globe, and a constellation of satellites carrying atomic clocks orbits the Earth. A radio signal from Cumbria, known as the MSF time signal, connects the atomic clocks at NPL to clocks all over the UK, enabling them to be in precise synchronisation.

How many times have you said "Just a minute”, when you have really meant half an hour? Does it really matter whether we know the time to such a small fraction of a second? You may question the relevance of the atomic time scale in your daily life, but in fact the influence of the atomic clock is everywhere. The clocks on aircraft, in mobile phone networks and on railway stations all rely on atomic clocks to ensure that they all tell exactly the same time. You may well have a radio-controlled clock at home or in school that picks up the MSF signal from Cumbria. You can use your PC to connect to the NPL atomic clocks over the internet to ensure your computer is synchronised with the national standard. Not only that, atomic time is necessary in banking, controlling air traffic, the emergency services and the Global Positioning System (GPS) for navigation.

Essen’s atomic clock took advantage of a property of atoms called hyperfine structure. The atom can be pictured as a mini solar system, with the heavy nucleus at the centre surrounded by electrons in a variety of different orbits. The orbits correspond to energy levels, and electrons can only move between levels when they absorb or release just the right amount of energy. This energy is absorbed or released in the form of electromagnetic radiation, the frequency of which depends on the difference in energy between the two levels. This transition is the source of the term "quantum jump", quantum referring to the tiny but precise amount of energy needed to allow the electron to jump to a different level. By measuring the frequency of the electromagnetic radiation, like counting the number of pendulum swings on a pendulum clock, we can measure the passage of time.

Although it is not the only element which could be used, the clock developed by Essen used caesium atoms. A beam of atoms is produced by evaporating caesium in an oven, and at this stage the atoms can be in either one of two energy levels, or states. The beam of atoms passes through a magnetic gate, which changes the direction of the atoms according to which of the two states they are in. The beam is then twice exposed to microwave radiation (in the Ramsey cavity - see Figure 3) with a frequency tuned to the energy needed for the electrons to jump between levels. A second gate filters out atoms in the wrong state while the rest continue to the detector to be counted. A feedback loop into an oscillator enables the microwave radiation to be tuned to the transition frequency when the number of atoms reaching the detector is a maximum. This frequency provides us with the “pendulum” of the atomic clock.

It may seem strange to create a clock based on atomic structure. Our own, human, concept of time is based on what we see around us - the rotation of the Earth on its axis defines the day, the time taken for the Earth to travel around the sun defines the year. Before the atomic clock the length of the day was determined by astronomical observation and the second was 1/86400 of the mean solar day, averaged over a year. However by the early 1900s these natural standards were becoming problematic.

The first NPL Caesium atomic clock with Louis Essen (right) and colleague Jack Parry in 1955

Observations of the movement of the moon led to the realisation that the speed of rotation of the Earth on its axis was slowing down. In addition to this relatively steady change, astronomers became aware that the earth was “wobbling” – unknown sources were causing unpredictable changes in the rotation of the Earth on its axis. As a result the length of the mean solar day was not constant, and therefore the accuracy of the second was limited by the stability of the Earth's rotation. A new standard was needed that would be independent of these fluctuations, and a standard based on the structure of atoms could provide just that, because all caesium atoms are the same and never change.

The move to atomic time was resisted by the astronomical community, but finally in 1967 international agreement confirmed that the caesium atomic clock would become the time standard for the world. This was not the end for astronomical time. Whilst the atomic clock counts seconds at a precise rate, the fluctuations of the Earth rotation mean that the time defined solely by an atomic standard would gradually drift away from the time as observed on Earth. This was overcome by defining Universal Coordinated Time (UTC). The length of the UTC second is determined by atomic clocks. Fluctuations in the Earth's rotation on its axis are then accounted for by adding or subtracting leap seconds to ensure that UTC remains within 0.9 seconds of the time based on the Earth’s rotation.

Being able to measure the discrepancies between the time as recorded by atomic clocks compared to the time recorded by astronomers has made it possible to investigate what could be causing these fluctuations. The unpredictable wobbles stem from a range of sources, including motion in the molten core of the Earth, tidal drag causing friction between the sea and the ocean floor and even variations in the weather. In 1983 a sudden lengthening of the day over the period of a few weeks was attributed to the El Nino phenomenon causing sections of the ocean to reverse their natural rate of flow and causing the Earth to slow down. High winds in India in the early 1990s had a similar effect. Using atomic time scales has made it possible to record and monitor these changes, and so increase our knowledge of complex geological and meteorological phenomena.

In theory it would be possible to have just one atomic clock, which could broadcast UTC time to the whole world via satellites. However there are risks. The clock may break down or need maintenance, so instead we could have two. But what if both break down or need maintenance? Or if the two clocks start telling different times, how do we know which one to trust? Factor in that few clocks operate continuously as they are upgraded and refined, and that all industrialized nations require their own national timescale, and the result is that the international time standard is maintained by 40 time laboratories around the world and is based on the average of some 260 atomic clocks. This diversity provides safety (a single clock in an earthquake zone would not be a good idea for example) accessibility, (each major industrial nation contributes to the time standard, and hence has direct access to the atomic clocks) and stability. A laboratory in France collates all the data, calculates the average and feeds back corrections to individual clocks as necessary.

Atomic clocks are not all earthbound. Nearly 100 atomic clocks orbit the Earth in a constellation of satellites which create the Global Positioning System (GPS). These are positioned so that at any time any point on the globe is within range of at least four satellites. A receiver only slightly larger than a mobile phone can pick up signals from the satellites and pinpoint it's location in 3D within an accuracy of a few metres by measuring the time it takes for a signal to travel from the transmitter to the receiver. The signal travels at the speed of light, so the receiver can calculate how far away each transmitter is and thus work out its location.

In the open sea and environments like the Arctic, with often limited visibility and a lack of permanent landmarks, these tools are vital for navigation. Other people making use of GPS include ecologists tracking the movements of rare animals, prospectors, surveyors, mountaineers, and the search and rescue teams required if any of these pursuits go wrong. Following the tsunami disaster in Asia, proposed warning systems could incorporate GPS to identify areas most at risk from future incidents.

The accuracy of GPS is so high that it requires adjustment for the effects of special and general relativity. Einstein’s equations tell us that two identical clocks will run at different rates if they experience different gravity. The clocks on satellites experience different gravity than their earthbound counterparts, and thus have to be adjusted to allow for relativistic effects. Onboard navigation systems in cars take advantage of GPS, so next time you are in car with this facility, remember general relativity could be helping you reach your destination!

Fifty years after the first atomic clock a new development has taken place in the form of trapped ion clocks. Ions are atoms that have gained or lost an electron, making them charged. Ion traps can isolate a single atom and then use a process called laser-cooling to cool the atom to just a fraction of a degree above absolute zero. Patrick Gill and his team at NPL have developed a strontium optical clock, so called because the transition frequency of strontium is in the optical region of the electromagnetic spectrum, as opposed to microwave frequency like the caesium clock. The caesium clock “ticks” at around 9 billion ticks per second, a clock using visible light can tick about 100 000 times faster. As Professor Gill says, "The more ticks we can count in a given period of time, the more accurately we can measure time". 
 

Operation of a caesium clock

Preliminary tests of the strontium optical clock referenced to NPL's caesium clock are looking positive, and indications are that such trapped ion clocks may have the potential
to measure time to one part in 1018 - nearly 1000 times more accurately than the best caesium clocks of today. These clocks could be installed in space-craft to allow precise navigation through deep space; to test effects of general relativity that at present are beyond our capabilities to measure; and even to investigate the fundamental constants of physics, to determine if they really are constant or whether they could change with time.

The atomic clock developed by Essen is now on display in the Science Museum in London. However the legacy lives on. Louis Essen probably wouldn't recognise the single ion atomic clocks of today, but the underlying concept of using the natural energy levels of atoms, the ticking of an atomic clock, is the same. Atomic clocks provide us not only with an accurate time scale, but are an essential part of mobile phone networks, assist in tracking rare animal species, and enable us to test the predictions of general relativity more rigorously. Essen’s physical atomic clock has now retired, but the future of atomic time is assured.

By Fiona Auty. She has a passion for communications especially science. Fiona is Head of Communications at the National Physical Laboratory where she oversees all external and internal corporate and scientific communications.